1. Field of the Invention
The present invention relates to diazonium salt and a synthesizing method thereof as well as a recording material. And more particularly, the present invention relates to diazonium salt that is useful as a synthesizing intermediate material of an azo pigment, an analyzing reagent, a raw material for a photosensitive recording material and the like, and a recording material that is superior in photo-fixing property, shelf-stability of raw photosensitive material and light-fastness of the surface of the background.
2. Description of the Related Art
Diazonium salts have been known as an important intermediate in synthesizing an azo pigment. Conventionally, for the synthesis method of the azo pigment, various methods have been known, and as described in Shin Jikken Kagaku Kouza (New Experimental Chemical Seminar), vol 14-III (Maruzen Co., Ltd.), pp.1516–1534, there are various synthesizing methods, such as a synthesis employing oxidation, reduction, substitution, addition, or condensation. However, such methods have the risk of an explosion of diazonium salt during synthesis, and there have been strong demands for the development of a stable diazonium salt that reduces the risk of an explosion.
As described in Japanese Patent Application Laid-Open (JP-A) No. 11-228517, the diazonium salt is used for a quantitative analysis of bilirubin that is a main component of the bile pigment contained in body fluid, and considered to be an important compound in medical and pharmaceutical fields as well.
In general, the diazonium salt, which has an extremely high chemical activity, and reacts with a compound referred to as a coupler having a phenol derivative and an active methylene base to easily form an azo dye, and also has a photosensitivity so that it is decomposed upon application of light to lose its activity. For this reason, the diazonium salt has been utilized for a long time as a photo-recording material typically represented by a diazo copying material (see Shasin Kougakuno Kiso: Higinenshasin hen (Basics of Photographic Engineering-Non-Silver-Salt Photography—), ed. Japan Photographic Society (Corona Publishing Co., Ltd., 1982), pp. 89–117, 182–201).
In recent years, the diazonium salt is also applied to a recording material capable of fixing an image by utilizing its characteristic of being decomposed by light to lose its activity, and the typical examples include a photo-fixing-type heat-sensitive recording material (proposed by Khoji Sato et al. in Gazou Denshi Gakkai Shi (Journal of Image Electronics Society) 11, no. 4 (1982): pp. 290–296) in which a recording material bearing a recording layer containing diazonium salt and a coupler is heated and allowed to react in accordance with an image signal to form an image, then the image is fixed with light irradiation.
These recording materials using the diazonium salt as a color-developing component has the disadvantage which resulting in a short shelf life as a recording material since the diazonium salt has an extremely high chemical activity, and is thermally decomposed gradually even in a dark place to lose its reactivity, Moreover, another disadvantage is that the generation of colored decomposed materials (stains) causes the non-image portion to be colored, since the residual diazonium salt compound is decomposed at a portion exposing the background that is a non-image portion, at photo-fixing. Furthermore, also in the finished image after the fixing process, the non-image portion has a poor light-fastness, and the coloring is enhanced when left under sun light or fluorescent light for a long time.
Improvement of the recording speed has been studied for a long time as a problem to be solved, and there have been strong demands for diazonium salt having an improved photo-fixing property and capable of carrying out a photo-fixing process for a short time.
As a means for solving such a problem of instability of the diazonium salt, various methods have been proposed. Among these, one of the most effective means is to encapsulate diazonium salt in microcapsules. By forming the microcapsules containing diazonium salt, the diazonium salt is separated from substances such as water and bases that accelerate the decomposition thereof, with the result that the decomposition is effectively reduced and it becomes possible to extremely improve the shelf life of a recording material using the diazonium compound (proposed by Tomomasa Usami, et al. in Densi Shasin Gakkai shi (Journal of Electrophotographic Society) 26, no. 2 (1987): pp. 115–119).
In the general method for encapsulating diazonium salt in microcapsules, diazonium salt is dissolved in a hydrophobic solvent (oil phase) and this solution is added to an aqueous solution (water phase) in which a water-soluble polymer has been dissolved, and emulsified and dispersed by a homogenizer or the like, while a monomer or a prepolymer to be used for forming a wall material of microcapsules is added to either the oil phase or the water phase, or to both of the phases so that a polymerizing reaction is allowed to take place in the interface between the oil phase and the water phase, or a polymer is deposited on the interface to form a wall made from the polymer compound; thus, microcapsules are formed. This method is described in detail in, for example, Asashi Kondo, Maikuro Kapuseru (Microcapsule) (Nikkan Kogyo Shimbun Ltd., 1970) and Tamotsu Kondo, et al. Maikuro Kapuseru (Microcapsule) (Sankyo Publishing Company, 1977).
The capsule wall of the microcapsules to be formed employs various materials, such as cross-linking gelatin, alginate, celluloses, urea resin, urethane resin, melamine resin and nylon resin.
In particular, in the case of microcapsules having walls having a glass transition temperature slightly higher than room temperature, such as those made of urea resin or urethane resin, the capsule wall is impermeable to substances at room temperature and permeable to substances at or higher than the glass transition temperature. Therefore, these microcapsules are referred to as thermal responsive microcapsules, which are effectively used as the recording material of a heat sensitive system.
In other words, in the case when a heat sensitive recording material is formed which has a heat sensitive recording layer comprising heat responsive microcapsules containing diazonium salt and a coupler placed outside the capsules as a main component of color-development, it is possible to maintain the diazonium salt stable for a long time, to easily form a color-developed image by applying heat thereto, and also to carry out a fixing process on the image thus formed by applying light thereto.
Therefore, it becomes possible to extremely improve the stability as the recording material by encapsulating diazonium salt into microcapsules.
As described above, although it has become possible to extremely improve the stability as the heat sensitive recording material, the instability in diazonium salt is not completely eliminated and the heat sensitive recording material and the like comprising the diazonium salt in the microcapsule have not achieved a sufficient long-term shelf life. Moreover, even after printing and fixing, when exposed to a light source for a long time, the photo-decomposing substance of the diazonium salt tends to have a photo-decomposing reaction, with the result that colored stains increase following the reaction, causing degradation in the degree of whiteness in the non-image portion (background portion) after the photo-fixing process and the subsequent reduction in the contrast with the colored portion.
Moreover, it has been known that the photo-decomposing reaction does not take place evenly to cause various decomposition products depending on ambient environments and the like; thus, among those matters of several tens of kinds or more, in particular, there are products having an absorbing range in a visible range, which are referred to as photo-decomposition stains. Here, when the stains are generated extremely, there is degradation in the degree of whiteness in the non-image portion (background portion) after the photo-fixing process, and the subsequent reduction in the contrast with the color-developing portion; consequently, the commercial value of the recording material is extremely impaired.
Here, since the photo-decomposition of the diazonium salt are complicated, and it is difficult to specify the resultant products, it has been considered that the reduction of the photo-decomposition stains is difficult.
Therefore, in recent years, with respect to improvements in the long-term stability by reducing the photo-decomposition stains, various researches have been made, and, for example, JP-A No. 8-324129 has proposed a photo-fixing-type heat sensitive recording material which is formed by using a specific hydrophobic oil in combination with microcapsules containing a photo-fixing-type diazonium salt so that it has a superior property in the raw shelf life, and is consequently less susceptible to degradation in the degree of whiteness even upon exposure to light for a long time after an image-forming process and also superior in image shelf-stability. Moreover, JP-A No. 11-78232 has proposed a non-fixing-type heat sensitive recording material using a novel diazonium salt in an attempt to improve the stability of the diazonium salt itself. In other words, a diazonium salt having the maximum light-absorbing wavelength in a shorter wavelength range than the proximity of 350 nm is contained in microcapsules, and by using a non-fixing-type heat sensitive recording material using these microcapsules, the proposed material can improve the degree of whiteness in the background portion and the image shelf life, after formation of an image with a light source having longer wavelengths than the proximity of 350 nm, generally exemplified by a general-use fluorescent lamp or the like.
However, at present, the raw material shelf life and the image shelf life in colored portions and background portions (non-image portions) after an image-recording process have been still insufficient, and there have been demands for further improvements in the stability.
Moreover, in recent years, there have been demands for shortened recording time required for the image recording process, that is, demands for high-speed image forming processes including printing and fixing processes. In particular, regarding the photo-fixing-type heat sensitive recording material using the diazonium salt, there have been strong demands for techniques that can achieve high-speed processes while improving the stability as described above, and in an attempt to satisfy the demands, it is essential to improve the photo-decomposing speed of the diazonium salt.
These recording materials using the diazonium salt as a color-developing component are generally arranged so that ultraviolet rays having wavelengths of approximately 360 nm are irradiated in the fixing process in order to effectively carry out the photo-fixing process. However, since the ultraviolet rays require a special light source and tend to give adverse effects to the eyes, there have been demands for a recording material using diazonium salt that can be effectively fixed by a light source that emits visible light rays having long wavelengths exceeding 400 nm.
However, recording materials using conventional diazonium salt raise a problem in that upon deactivating the diazonium salt by using a light source having a long wavelength of 400 nm or over, the fixing process becomes slower and takes a long time. Moreover, when a photo-fixing process is carried out for a long time in an attempt to completely carry out the fixing process, generated products by the fixing process are allowed to further react to cause degradation in the degree of surface whiteness in a color-developed image.
Moreover, in JP-A No. 11-116553 and Japanese Patent Application No. 2002-131575, the inventors, etc. of this invention have proposed a novel diazonium salt having the maximum absorption wavelength (λmax) over 400 nm (that is, 4-indolino-type diazonium salt in which a substituent on an aromatic ring of a heterocyclic structural portion containing nitrogen, is a hydrogen atom, an alkyl group, an alkoxy group, a halogen group, an acyl group or an acyl amino group); however, in the heat sensitive recording material comprising the diazonium salt as a color-developing component, there have been further demands for improvements particularly in the prevention of coloring of background portion after the photo-fixing process.